Numerical investigation into the loading behaviour of filters operating in the diffusional and interception deposition regimes

Abstract

Using a previously developed theory, which allows for changes in gas flow from deposit growth, calculations are performed for deposition on an initially cylindrical fibre. The deposit is given a specified porosity through which the flow is calculated from Darcy's equation using the Beavers and Joseph (1967) boundary condition at the outer boundary. Results are obtained for different porosities ϕ, the volume fraction of space in the deposit, for flow conditions such that deposition occurs by the mechanisms of diffusion and interception, but not impaction (Stk < 1). Dependencies given in the literature of the deposit permeability on ϕ are examined. The dominant mechanism is determined by the value of the parameter, s = R / δ, where R is particle to fibre radius ratio and δ is the non-dimensional thickness of the flow diffusion layer.Where diffusion dominates, s < 1, increase in porosity does not significantly increase deposition, apart from the effect of its lower density. Where s > 1 and interception dominates, deposition increases with increase in ϕ and is significant for ϕ ≥ 0.9 where more streamlines pass through the deposit. The shape of the deposit remains peaked at the front of the fibre, at the forward stagnation point, but, if a deposit has an initial flat front, it grows into a shape peaked away from the stagnation point, as observed by Kanaoka et al (1986). Possible reasons for this behaviour are discussed.

Modeling of dynamic deposition and filtration processes of airborne particles by a single fiber with a coupled lattice Boltzmann and discrete element method

“…With the SFE information and cell dimensions, the collection efficiency of a filter can be estimated using many classical expressions [18]. During the simulation of particle filtration by an individual fiber, most of the models assume that once a particle is colliding with the fiber or with the deposit, it is captured immediately and frozen [2,3,5,7]. Indeed, the particle rebound effect is weak for submicrometer adhesive particles, thus the "immediately capture" is reasonable.…”

Dynamics of particle loading in deep-bed filter. Transport, deposition and reentrainment

“…This approach can be efficiently used only for the initial stage of filtration, when previously deposited particles have not as yet significantly changed the fluid flow field cpe.czasopisma.pan.pl; degruyter.com/view/j/cpe and surface open for deposition. Some papers (Dunnett and Clement, 2006;Dunnett and Clement, 2012) take into account deposit growth, but the approach requires making assumptions of deposit structure. The important advantage in deep bed filtration modeling was introduction of lattice gas automata (Biggs et al, 2003) and lattice-Boltzmann method (Long and Hilpert, 2009), that allows to take into account the geometry of flow change due to deposition of suspended particles.…”

Critical trajectories for aerosol particles: Their determination for impaction in fibrous filters and in oscillating bubbles

“…In our previous work on filtration ( Dunnett and Clement 2006, 2009and 2012, the boundary element method was used to calculate the flow field for aerosol particles incident on a fibre. The deposition mechanisms examined for a growing deposit were diffusion ( Dunnett and Clement 2006) and also interception Clement 2009, 2012 ).…”

“…The determination of particle trajectories by solution of their equation of motion in a given flow field was discussed by Davies (1973), and here we describe a very efficient method for reaching the limiting critical trajectory for impaction in a fibrous filter by a process of iteration. We also describe a direct method for their determination applicable to impaction in an oscillating gas bubble inside a liquid in which the equations in a time-dependent velocity field can be transformed into an integral equation.In both cases the flow field in which the particles move must be known or calculable, and we assume the particles are subject to Stokes' drag in conditions such that the Stokes number, St so that diffusion can be neglected in describing the particle motion.In our previous work on filtration ( Dunnett and Clement 2006, 2009and 2012, the boundary element method was used to calculate the flow field for aerosol particles incident on a fibre. The deposition mechanisms examined for a growing deposit were diffusion ( Dunnett and Clement 2006) and also interception Clement 2009, 2012 ).…”